1. Field of the Invention
The invention relates generally to a low free formaldehyde phenolic resin composition in particular for use in abrasive products. The invention further relates to abrasive products comprising abrasive particles fixed in a resin, in particular coated abrasives and molded abrasives, and to a method for manufacturing the phenolic resin composition and to processes for making said in abrasive products.
2. Description of the Related Art
Coated abrasives are flexible products, used in forms of sheets, belts, mop discs, flap discs etc. These products are used for sanding plywood and fiber boards, wood and paints, grinding and finishing metal, glass, plastics etc. The abrasive grains are fixed in a single layer by a binder on a backing material, which can be paper, natural or synthetic fabrics vulcanized fibers and others. The main binders are phenolic resins, urea resins and also animal glue is still in use.
Bonded abrasives are grinding tools, where the abrasive grains are fixed by a binder in a defined shape like wheels, segments, blocks etc. We can distinguish between two main types of binder, vitrified or resin bonded. For the resin bonded grinding tools mainly phenolic resins are used, with and without fillers and additives. These products are used for sanding, grinding, cutting and polishing. The choice of the resin for the binder in general depends on the application of the abrasives. Mainly phenolic resins are used, when heat resistance a high grinding performance like high stock remove and long lifetime are required. For less demanding performance urea resins and also animal glue are used. Phenolic and urea resins have the disadvantage of a relatively high formaldehyde emission. For special purposes, such as wet grinding, epoxy, urethane or alkyd resins are applied.
Because of the good performance in abrasive products, the demand in abrasive products based on PF resins is increasing more and more. A problem of PF resins is that they contain relatively high amounts of unreacted free phenol and free formaldehyde monomers, which poses safety, health and environmental problems. Low free phenol content has been in demand already for several years for environmental reasons. In the last 2 years it has also become a requirement to have low free formaldehyde content, because formaldehyde has been considered carcinogenic by the IARC. The combination low free phenol and low free formaldehyde by maintaining specific resin properties is quite difficult to achieve. Low free phenol is preferably less than 10% and typically between 2 and 5 weight %, and low free formaldehyde is preferably less than 0.5 weight % and typically less than 0.1 weight %. The usually applied formaldehyde scavenger additives like urea and melamine are limited in their amount of addition, because higher amounts can cause precipitation, reduced storage stability and processing problems of the resin.
WO2009073703, describes a phenolic resin for use in coated abrasive products having a relatively low free formaldehyde content formulation formed by reacting a basic catalyst, formaldehyde, water and a phenol to form an intermediate composition to which an alkanolamine is subsequently added. The alkanolamine is added in excess to react away the formaldehyde. Further, alkanolamine has a strong negative impact on water tolerance and increases volatiles in the resin.
Thus the problem to be solved is to provide a process of manufacture of a resin composition having further reduced free formaldehyde and low free phenol content without significant reduction of resin properties, storage stability and performance in the envisaged application in abrasive products.